Q.4 Phenylketonuria is a type of
1. X-linked dominant disease
2. X-linked recessive disease
3. autosomal dominant disease
4. autosomal recessive disease
Phenylketonuria Inheritance: Autosomal Recessive Explained
Phenylketonuria (PKU) is a genetic disorder caused by mutations in the PAH gene on chromosome 12. The correct answer to the multiple-choice question is 4. autosomal recessive disease.
What is Phenylketonuria?
Phenylketonuria, or PKU, results from deficient phenylalanine hydroxylase enzyme activity, leading to phenylalanine buildup in blood and brain damage if untreated. Newborn screening allows early dietary management to prevent intellectual disability. It affects about 1 in 10,000-15,000 births worldwide.
Correct Answer: Autosomal Recessive
PKU requires two mutated gene copies (one from each parent) for symptoms to appear, confirming its autosomal recessive inheritance on non-sex chromosome 12. Carriers (one mutated copy) show no symptoms but have a 25% chance per child of passing PKU if both parents are carriers. This pattern matches classic recessive traits like cystic fibrosis.
Option Explanations
| Option | Description | Why Not PKU? |
|---|---|---|
| 1. X-linked dominant disease | A single mutated gene on the X chromosome causes the disorder; affects males and females differently, often more severely in males (e.g., hypophosphatemic rickets). | PKU gene is on autosome 12, not X; equal male-female incidence rules out X-linkage. |
| 2. X-linked recessive disease | Requires two mutated X genes in females, one in males; common in males (e.g., hemophilia, color blindness). | PKU shows no sex bias and autosomal location; not X-linked. |
| 3. Autosomal dominant disease | One mutated autosome copy triggers disorder; 50% inheritance risk (e.g., Huntington’s disease). | PKU needs two copies; carriers are asymptomatic. |
| 4. Autosomal recessive disease | Two mutated autosome copies needed; matches PKU perfectly. | Correct: Confirmed by genetic studies and screening data. |
Why Understanding Inheritance Matters
Knowing PKU’s autosomal recessive nature guides genetic counseling and screening, vital for at-risk families in regions like India with high consanguinity. Early detection via heel-prick tests prevents irreversible damage through low-phenylalanine diets.
